Lightroom is installed on an SSD drive and the catalog file is located on the same SSD drive as well.

The SSD drive has a lot of space free (at least 10 GB).

I'm using a Dell U2711 monitor (native resolution 2560x1440).

The most interesting thing is that if I adjust lightroom window size, then the development sliders work very fast.

LR is a non-destructive editor that never writes or changes the original file. PS is a destructive pixel editor that also caches multiple copies of the edited file in memory to speedup "undo" image building. PS4-6 supports Open GL, which uses the graphics processor for processing certain image views (Rotate, 3D, etc.). LR and PS work entirely differently concerning onscreen image building, and LR benefits very little by increasing graphics card performance.

A faster system processor will make LR more responsive with a high resolution display, but simply keeping the Loupe image smaller when adjusting sliders in the Develop module will do the same thing. My system is only slightly higher performance (i7-280, 12GB) and I have no issues. BUT my monitor is 1920 x 1080 (~2Mp) versus 2560 x 1440 (~3.7Mp). Your processor needs to build almost twice (2x) the number of pixels every time you make a change in the Develop module. If my i7 system performance is about 1.5x your i5 system, then screen response time will be almost three times faster ( .3 to .5 Sec. versus 1.0 to 1.5 Sec.).Using dual high resolution displays is even more challenging for LR.

Try making the Develop module side panels as large as possible by dragging the border with your mouse, and perhaps increase the height of the filmstrip. Then use CTRL + SHIFT + F Key to enter and exit full screen mode for a larger image review of your Develop setting changes. On my system this takes about 1.0 sec., so this may not be a time saving option for you (i.e. ~3.0 sec.). Give it a try.

I am with you on the sluggishness of the develop slider activation... I'm sitting on a .5 sec. delay on a 4.5Ghz six-core with 32GB RAM & SSD drives...

One tweak you can use if you have a tablet is to play off of the fact that while the develop sliders are inactive for a little period while the photo renders in the develop module, the -/= keyboard buttons are active and they can control whatever develop slider is activated (you activate a slider by clicking on its name). So, if the table has a little dial thing (eg. wacom tablet) you can set it to control the -/= buttons and have instantaneous control over the exposure slider for example. While you still have to wait for the other controls to become active, at least you have one slider to play with :-)